The invention relates to oscillator circuits and in particular, to FM local oscillator circuits with automatic frequency control (AFC) that are suitable for integrated circuit (IC) fabrication.
In the present state of the broadcast radio receiver art, substantial progress has been achieved in fabricating much of the AM and FM circuitry in monolithic IC form, including the AM mixer and AM local oscillator, the IF amplifier, the AM and FM detectors and the audio amplifier. Examples of this form of circuitry may be found in U.S. Pat. No. 3,936,750, entitled "AM-FM Receiver Having Improved Bias Supply Circuit", William Peil and Robert J. McFadyen, issued Feb. 3, 1976; U.S. Pat. No. 3,999,138, entitled "Detector for AM-FM Signals", William Peil and Robert J. McFadyen, issued Dec. 21, 1976; and U.S. Pat. No. 4,025,871, entitled "Audio Amplifier for Integrated Circuit Fabrication Having Controlled Idling Current", William Peil, issued May 24, 1977. However, a number of difficulties have thus far existed with respect to adapting the FM mixer and FM local oscillator circuit to IC fabrication. In particular, difficulties regarding IC adaptation of the FM local oscillator relate primarily to the relatively high frequency of operation of these circuits normally requiring a high frequency IC processing, and the need to exhibit a sensitivity to automatic frequency control, which is a desirable property for FM operation. As a result, prior to the present invention FM local oscillator circuits have not been integrated with those radio receiver circuits previously fashioned in monolithic IC form.